Interstate 405 (Washington)

I-405 begins as a continuation of State Route 518 (SR 518) at a junction with Interstate 5 in Tukwila, northeast of Seattle–Tacoma International Airport. It gains a set of high-occupancy vehicle lanes (HOV lanes) and travels east around the Southcenter Mall and Tukwila's retail district as an eight-lane freeway. I-405 crosses the Green River and intersects Interurban Avenue and SR 181 near the Tukwila train station and Starfire Sports complex. The freeway enters the city of Renton and travels between a waste treatment plant and the former Longacres racetrack (now a Boeing office park),[3] crossing over a major railroad used by BNSF and Union Pacific. After crossing the Black River and passing through suburban office parks, the highway intersects SR 167 (the Valley Freeway) and Rainier Avenue in a cloverleaf interchange and begins to turn north towards downtown Renton.[4]

The freeway enters a series of "s-curves" as it approaches downtown Renton,[5] turning north after intersecting SR 515 at Talbot Road in a half-diamond interchange and continuing northeast along the underside of Renton Hill.[6] After crossing the Cedar River near the Renton Public Library, I-405 passes through a public park and intersects SR 169 and SR 900 at two separate interchanges, beginning a short concurrency with the latter. The freeway passes the Kenworth truck plant and the Renton Landing shopping center near the Boeing Renton Factory before reaching a second junction with SR 900, which continues east towards Issaquah.[4]

Climbing Kennydale Hill,[5] I-405 intersects N 30th St and Kennewick Place NW, which serves north Renton, and Kennydale, before quickly descending to another hybrid diamond/partial cloverleaf interchange with NE 44th Street, which provides access to Newcastle. I-405 now closely follows Lake Washington, and enters a wooded section until an interchange with Lake Washington Boulevard and 112th Ave SE which serves both a park and ride station and the community of Newport Hills. The final interchange before I-90 is a diamond interchange serving Coal Creek Parkway.

I-405 heading into downtown Bellevue

Factoria Mall is east of I-405 as it approaches a full stack interchange with Interstate 90 which provides access to Seattle, Mercer Island, as well as the Eastgate neighborhood of Bellevue and Issaquah. Mercer Slough is west of I-405 as it heads towards downtown Bellevue. A 2008 construction project demolished the Wilburton Tunnel which formerly covered a portion of the freeway between Factoria and Bellevue.[7]

I-405 passes the historic Wilburton Trestle near a diamond interchange with SE 8th Street that connects the freeway to South Bellevue and the Lake Hills area. In quick succession is a short diamond interchange with NE 4th Street, a full cloverleaf with NE 8th Street, serving downtown Bellevue and Overlake Medical Center, and an HOV only left exit to the Bellevue Transit Center at NE 6th Street. The freeway's set of four high-occupancy toll lanes (HOT lanes) begin at the NE 6th Street ramp, traveling north with limited entry and exit points. SR 520 which provides access to Seattle, Medina, the Microsoft campus, and Redmond, is the next interchange, with a half cloverleaf interchange that includes several fly-over ramps to allow complete access to and from SR 520.

Bridle Trails State Park borders the freeway on the east side, while Watershed Park and Northwest University are on the western side of the freeway as it approaches NE 70th Place, serving Houghton and Snyders Corner before approaching a full cloverleaf with former SR 908 serving Kirkland. An incomplete interchange with NE 116th Street comes before I-405 connects to Totem Lake via an interchange with NE 124th Street and an HOV only left exit to the Evergreen Hospital and NE 128th Street. NE 160th Street is the next interchange before I-405 crosses over the Sammamish River and connects to SR 522 which provides access to downtown Bothell and Woodinville. An interchange with NE 195th Street provides access to the local business park and the University of Washington Bothell branch.

I-405 in Snohomish County

Entering Snohomish County at mile post 25, I-405 enters a wooded area before coming to an interchange with the Bothell-Everett Highway (SR 527), which is the final interchange before I-405 terminates at an interchange between Interstate 5 and SR 525.[4]

As of 2007, the most heavily traveled section of I-405 is near SE 8th Street in downtown Bellevue, with a daily average of 200,000 cars per day traveling through this segment.[8] This number was slightly lower in 1997, with 190,000 cars per day,[9] but has dramatically increased since the first measurement in 1967, with 32,600 cars per day traveling through Bellevue.[10]

The first road connecting most of the eastside communities, Secondary State Highway 2A (SSH 2A),[11] was already in existence since at least 1927. SSH 2A connected US 10 and US 410 in Renton to the north, through Kirkland and Bothell, connecting back to US 99 in Everett.[12] In 1964, the system of Primary and Secondary state highways was removed in favor of signed routes, and PSH 2A was renumbered SR 405. The first segment of the interstate to be completed was between Tukwila and Renton, opening to traffic on August 31, 1965.[13] By 1971, SR 405 was renumbered to Interstate 405 after it was completely improved to Interstate Highway standards.

One of the major construction projects on the highway was straightening the s-curves in Renton. This project began in 1990, and was estimated to cost $70 million. Also during this time the portion of the highway between I-5 and South Renton was being repaved and HOV lanes were being added.[14]

On September 27, 2015, a set of high-occupancy toll lanes (HOT lanes) opened on I-405 between Bellevue and Lynnwood, replacing the existing HOV lanes from NE 6th Street to I-5/SR 525.[15][16]

• Media related to Interstate 405 (Washington) at Wikimedia Commons
• WSDOT I-405 Corridor Program